Fiber reinforced plastic (FRP) is a composite material used in civil and architectural engineering fields, transportation fields such as vehicle materials, electronic/electrical fields, aerospace fields, among others. In such an FRP, a thermosetting resin, such as an unsaturated polyester resin, a vinyl-ester resin, an epoxy resin, a phenolic resin, or a thermoplastic resin such as polyethylene, polypropylene, ABS resin, polycarbonate, polyacetal, polyamide, polystyrene, polyphenylene sulfide, etc. is used as a matrix resin. An inorganic fiber such as a glass fiber, a metal fiber, a ceramic fiber, a carbon fiber, etc., or an organic fiber such as a natural fiber, a polypropylene fiber, a polyamide fiber, a polyester fiber, a polyacrylate fiber, a polyimide fiber, etc. is used in various forms as a reinforcing fiber. Among the reinforcing materials, glass fibers have been widely used in consideration of strength of a resulting FRP. For example, when an inorganic fiber such as glass fiber or an organic fiber different from a material of the resin matrix is used as the reinforcing material of FRP, there is a disadvantage in that recycling is difficult.
Accordingly, FRP with high recyclability, which is composed of the resin matrix and the reinforcement being made of identical materials, has been developed. With respect to such self-reinforced composite, FRP using an organic fiber, in particular, a resin fiber rather than an inorganic fiber as the reinforcing material has been used. This FRP has slightly lower strength than those composed of inorganic fibers. However, there are many advantages because the reinforcing fiber and the matrix resin have similar physical properties. The self-reinforced composite is a low specific gravity, high-strength material having a very low specific gravity of 0.9 or less, but it may exhibit tensile modulus of elasticity and strength being similar to those of the existing glass fiber (discontinuous)-reinforced composite. Therefore, when the self-reinforced composite is used as an alternative to the existing short/long GFRP, there are advantages that additional weight reduction of about 30% is possible and the self-reinforced composite may exhibit excellent recyclability owing to use of the single thermoplastic material, compared to the existing fiber-reinforced composite.
FRP composed of the matrix resin and the reinforcing material being made of the identical materials may be prepared by a method of blending or laminating the matrix resin and the reinforcing material, in which the reinforcing material is composed of a thermoplastic fiber or film with high strength and high modulus of elasticity, or the matrix resin is composed of the material identical to the thermoplastic resin used in the reinforcing material and a solvent, and then heating and pressing them to form a composite. However, this method generates a problem of environmental pollution due to use of a solvent. Accordingly, there is a need to provide a high-strength FRP with excellent recyclability and an improved preparation method thereof.
There have been many attempts to improve a reinforcing effect and tensile property of FRP, which is composed of the matrix resin and the reinforcing material made of the identical material and shows a lower reinforcement effect than those composed of inorganic reinforcing material.
For example, U.S. Pat. No. 6,458,727 discloses a method of preparing a self-reinforced composite, in which a highly drawn polypropylene tape is prepared and then only the surface thereof is selectively melted. However, physical properties may be deteriorated due to generation of waviness during tape weaving, and a processing window becomes narrow because only the surface of the highly drawn polypropylene should be selectively melted, leading to a reduction in productivity.
Another example is a self-reinforced tape which is prepared from homopolypropylene (core) and random-polypropylene (shell) by co-extrusion as in the manufacture of Pure (trade name) by Lankhorst. In this product, the used materials are the same polypropylene, but the random polymer having a melting point lower than that of the resin used in the core is applied to the surface, thereby widening the processing window and increasing productivity. However, an additional process of weaving this self-reinforced composite tape is needed, and a deterioration of the physical properties due to generation of waviness during tape weaving still remains problematic.
Accordingly, there is a demand for a self-reinforced composite with excellent physical properties, such as tensile strength, which is prepared by a simple process with high productivity, and a preparation method thereof, in which deterioration of the physical properties due to generation of waviness during tape weaving is reduced, heat-bonding processes are simple, and high productivity is achieved, resulting in reduced deterioration of the physical properties.